1. Field of the Invention
The invention relates to a method and apparatus for determining a path difference in a Michelson interferometer which comprises a mirror element reciprocatable by means of a drive as well as a detector and an analog to digital converter, and which is provided for determining an electromagnetic spectrum.
2. The Prior Art
An interferometer constructed in this manner and used for the aforementioned purpose is known for example from U.S. Pat. No. 4,799,001. Furthermore, from DE 35 03 116 A1 it is known to use for determining paths and path differences length measuring systems which as optical storage medium comprise a glass scale in the form of a main scale and a scanning head. The main scale is provided parallel to the travel path and in operation a pulse sequence corresponding to a sort of clock track is derived and furnishes a clock pulse as well as path information.
In interferometers of the type described above, at present a path measurement is carried out by means of a laser radiation. The radiation of the laser passes through the interferometer and the path differences are apparent from the resultant interference signal using the knowledge of the wavelength of the laser radiation. A disadvantage with this type of path measurement is that either a reference interferometer for the laser radiation must be coupled to the signal interferometer or the laser radiation must be supplied through the signal interferometer. In the second solution a separate range of the beam splitter must be suitable for the laser radiation. This either restricts the range of the signal radiation or requires the use of larger components, i.e. larger beam splitter, mirrors, lenses, etc.
Furthermore, it is difficult to make a beam splitter which has different optical properties in two geometrically separate regions because the signal radiation usually lies in the infrared range whilst the laser is usually a HeNe laser with a wavelength of 632.8 nm.
Another disadvantage is that the sampling interval in the digitizing is predefined by the wavelength of the laser radiation. The sampling theorem requires a sampling interval which is somewhat less than half the shortest measuring wavelength. If the latter is in the range of integer multiples of the laser wavelength or somewhat therebelow, the sampling increment width must be one half smaller than is actually necessary because it is derived from the laser interferogram and the latter does not permit any finer gradations. As a result, almost twice as many data points than necessary must be recorded and processed. In addition, the bandwidth then is twice as great as necessary and thus the signal to noise ratio achieved is poorer than that theoretically obtainable. In other words, the dimensioning depends on the laser and not on the radiation to be investigated, which would be the optimum case.
It is also disadvantageous that economic laser tubes as a rule have a short life and tubes with long lives are expensive.